Is it sensible to simply delete all mail that SpamAssassin marks as being spam?

In general, no. While SpamAssassin is very good at picking out a large proportion of spam, it's impossible for a computer to do this job perfectly. You should only delete mail if you (and your users/customers) would find it acceptable to lose mail that might be legitimate. A much better idea is to filter possible spam into a separate folder that can be checked less frequently than the normal mailbox. It is possible to reject the mail at the smtp level, generating a delivery error, so the sender is notified that their message is rejected. This works well imo. You need to use a mail server that supports this(I use mimedefang+sendmail). Mimedefang also allows me to save the mail to a central archive that I can extract from if I get a FP. If you do reject mail at the 5xx delivery level you need to set your spam threshold higher than the default of 5.

If you have so much spam that manually reviewing all messages isn't practical, you can reduce the damage by creating rules to implement a whitelist. You can also implement through rules an "email password"; email that includes the email password in the subject (or body) could be ranked as much less likely to be spam. Then, make sure that those who might legitimately contact you can learn the email password, e.g., placing a shrouded graphic of the email password on your website. Make sure you can can change your email password later (e.g., by changing rules), in case spammers start including your old email password. You can see more about email passwords at http://www.dwheeler.com/essays/spam-email-password.html

(EditHint: I log in into a shell account, so I wrote a perl script that checks my spam mailbox for new messages [i.e. those missing a Status field in the header] and included it in my .bashrc. This script, with the -r option, also marks all those messages as read so I don't see those same headers popping up next time I log in.)

But I really really want to do it anyway!

Don't say we didn't warn you

This page – "So you want to delete all your spam!" http://www.exit0.us/index.php/So%20you%20want%20to%20%27delete%27%20all%20your%20spam%21 tells how to configure procmail + SpamAssassin to delete email that looks like spam.

(EditHint: should we merge these 2 wiki pages, reducing one to merely a pointer to the other ?)

If you do want to delete mail tagged as spam, you really need to consider where you put the borderline. By default, SpamAssassin will call all messages scoring over 5.0 spam. If you set the "delete" threshold at 5.0, there's a real risk that you'll delete legitimate mail that happened to score slightly over 5.0. You should consider setting a deletion threshold at something considerably higher than 5.0 – maybe as high as 10 or 15 points.

It's not too difficult to write a non-spam message that scores 6.0. But it's quite hard to accidentally write a message that scores over 15! On the other hand, spammers seem quite capable of writing messages that score 30 or more points.

You really do need to make sure your users are aware of any policy that will result in messages being deleted without them knowing about them. You might possibly need to be sure that doing this is permissible under the laws that apply in your jurisdiction.

If you're working in an organisation that might get upset if legitimate messages "go missing", you might need to get an auto-delete policy signed off by management, just to ensure you are covered when the mail that clinches a multi-million deal gets auto-deleted because it scored 5.1 when the auto-delete threshold was set at 5.0.

How can I configure procmail to bounce messages flagged as spam?

Don't bounce to the 'From:' addresses. They are almost always forged in spam messages. The innocent people at those addresses don't need thousands of bounce messages. If your own address has been forged by spammers as From-address, you know what I'm talking about.

(EditHint: Are you saying don't bounce at all ? Or is there an envelope-from that's different from the "From:" in the subject line that I *can* bounce to ?) Reply: don't bounce spams at all. You can't trust any portion of the message, including the envelope info. If you bounce spam, you're just contributing to the effectiveness of Joe Jobs. The rest of this page is in error. The only way to block spam is to deny it before your mailer accepts it for delivery. Once you've accepted it, all you can do is delete it.

Here's a recipe that I use with qmail and maildrop (if you use e.g. sendmail and procmail instead, it will not work) to delete and bounce all spam messages:

{{{$ cat .qmail

This recipe is for one of my users who is drowning in hundreds of spam messages per day, and thus does not have time to weed through a spam folder. In case someone sends a legitimate message to her and it gets flagged as spam, they will receive a bounce message and thus be aware of the situation.